Transient spectroscopy is used to study the kinetics of conformational changes in macromolecules subsequent to excitation with a pulsed laser. Changes in both the tertiary and quaternary structure of hemoglobin have been observed following the photodissociation of carbon monoxide from the hemes. The kinetics of ligand rebinding have also been studied in molecules which can be switched to the low-affinity 'T' state with ligands bound. The results show that the binding rate is reduced at least 30-fold in the photolytically produced deoxyhemoglobin molecule within approximately 20ns after the iron ligand bond is broken. Transient spectroscopy has also been used to study the photoproduct cycle of bacteriorhodopsin, a molecule which pumps protons across the bacterial membrane. Our first studies have shown that the light-adaptation mechanism probably results from formation of multiple products in the photolysis step. In addition the data strongly suggest branching in the photocycle of the light-adapted molecule, in contrast to the conventional Br -more than K -more than L -more than M -more than 0 -more than Br photocycle.